1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser, and more specifically to a distributed feedback semiconductor laser which outputs a beam of a single wavelength for such purposes as optical communication and optical measurement.
2. Description of the Related Art
As shown in FIG. 21A, a conventional semiconductor laser which outputs a beam for use in optical communication and optical measurement comprises, for example, a semiconductor substrate 1, on which an active layer 2 and a cladding layer 3 are grown epitaxially.
In such a semiconductor laser, a beam 4 is outputted from each facet of the active layer 2 when a predetermined voltage is applied between a lower surface of the semiconductor substrate 1 and an upper surface of the cladding layer 3.
In close examination, as shown in FIG. 21B, the beam 4 can be regarded as a set of beams each having a slightly different wavelength λ from others.
In order to output the beam 4 of a single wavelength λ0, proposals have been made for a distributed feedback (DFB) semiconductor laser as shown in FIG. 21C, which includes a diffraction grating 5 formed near the active layer 2 and in a direction in which the beam 4 is outputted.
In a DFB semiconductor laser having the diffraction grating 5 incorporated therein, assuming that n is an effective refractive index of an optical waveguide, and d is a grating pitch, of a beam having many different wavelengths λ and generated from the active layer 2, a beam 4a having a single wavelength λ0 that satisfies the condition of wavelength λ=2nd should be outputted.
However, according to this DFB semiconductor laser, if the diffraction grating 5 formed inside is of a phase continuous type, being uniform in the beam outputting direction as shown in FIG. 21C, resulting oscillation is not genuine “single-mode oscillation” based on the single wavelength λ0 in principle.
Specifically, in this case, as shown in FIG. 21D, the beam having the wavelength λ0 that satisfies the condition of λ=2nd is not outputted but rather, the outputted beam is of two wavelengths λ+1 and λ−1, i.e. values sandwiching the intended wavelength λ0.
In an attempt to eliminate such a problem and to realize the “single-mode oscillation,” a phase shifting structure called λ/4 shifting structure is formed at a midpoint of the diffraction grating 5 so that phase of the beam is shifted as much as λ/4.
However, the diffraction grating 5 having the phase shifting structure at a midpoint thereof cannot be manufactured in batch exposure process by using holographic lithography method which is simple and advantageous for mass manufacture. For this reason, manufacturers employ a manufacturing method of drawing for a long time using electron beam lithography.
On the other hand, the Japanese Patent No. 1781186 (Jpn. Pat. Appln. KOKOKU Publication No. 4-67356, Jpn. Pat. Appln. KOKAI Publication No. 60-192378) discloses a DFB semiconductor laser that uses a diffraction grating formed by the holographic lithography method yet has a performance equal to that achieved by the gratings having the λ/4 shifting structure.
Specifically, in this DFB semiconductor laser, as shown in FIG. 22, the active layer 2 has a lower surface formed with first and second diffraction grating waveguides 6a, 6b and a flat connecting waveguide 7 connecting the first and second diffraction grating waveguides 6a, 6b as an integrated structure on the same plane.
Each of the diffraction gratings in the first and second diffraction grating waveguides 6a, 6b is formed by matching respective phases so as to serve as part of one virtual diffraction grating by using the holographic lithography method.
The connecting waveguide 7 has a specific propagation characteristic. Specifically, as compared to the case where the connecting waveguide 7 has the same structure as the first and second diffraction grating waveguides, the phase of the passing beam is shifted from π multiplied by an integer.
Note that the related document describes that according to this DFB semiconductor laser, “phase of the beam passing from the first diffraction grating waveguide 6a to the second diffraction grating waveguide 6b is shifted from π multiplied by an integer”. It is obvious, however, that a manufactured DFB semiconductor laser has a higher probability of outputting a beam at a single wavelength when “the phase of the beam passing from the first diffraction grating waveguide 6a to the second diffraction grating waveguide 6b is shifted from π multiplied by a half value of an odd integer (π/2, 3π/2, 5π/2, . . . )”.
However, the DFB semiconductor laser having the structure shown in FIG. 22 still has the following problems yet to be solved.
First, discussion will be made for the length L of the connecting waveguide 7 which is a function of shifting the phase of the beam that passes from the first diffraction grating waveguide 6a to the second diffraction grating waveguide 6b from π multiplied by an integer.
In general, in the connecting waveguide 7, the phase of a beam traveling on the connecting waveguide 7 is shifted as compared to the case in which there is a diffraction grating placed instead. This is because the structural difference in the optical waveguide, i.e. presence or absence of the diffraction grating, makes a slight difference in the beam's propagation constant which corresponds to the propagation velocity of the beam.
In other words, the beam's propagation constant is determined by an effective refractive index n which the traveling beam is sensitive to.
Referring now to FIG. 23, the difference between the propagation constant of the beam traveling on the diffraction grating waveguides 6a, 6b and the propagation constant of the beam traveling on the connecting waveguide 7 depends upon a difference (n0−n1) between an effective refractive index n1 of the connecting waveguide 7 and an average effective refractive index n0 of the diffraction grating waveguides 6a, 6b. 
Further, the difference (n0−n1) in the effective refractive index depends upon a difference (h0−h1) between a thickness h1 of the connecting waveguide 7 and an average thickness h0 which is derived while taking into account a depth of grooves existing between the respective diffraction gratings of the diffraction grating waveguides 6a, 6b. 
In other words, in order for the amount of phase shift of the beam that travels on the connecting waveguide 7 to be close to π multiplied by a half value of an odd integer (π/2, 3π/2, 5π/2, . . . ), accurate control must be made on the difference (h0−h1).
Each of the gratings in the diffraction grating waveguides 6a, 6b of this DFB semiconductor laser is generally formed by etching.
Specifically, the average thickness h0 of the diffraction grating waveguides 6a, 6b depends upon the depth of grooves formed by etching, and therefore, depends upon accuracy of the etching performed in the process of manufacturing the DFB semiconductor laser.
As a result, it is impossible to provide accurate control on the amount of the phase shift of the beam that travels on the connecting waveguide 7, and there is a problem that the manufactured DFB semiconductor laser has a low probability for oscillation at a single wavelength, which means that product yield is low in the manufacture.
Further, since the thickness h1 of the connecting waveguide 7 is constant, and the average thickness h0 of the diffraction grating waveguides 6a, 6b is also constant, the difference (h0−h1) is constant.
As a result, the length L of the connecting waveguide 7 is determined to a substantially fixed value for the amount of phase shift of the beam that travels on the connecting waveguide 7 to have a value equal to π multiplied by a half value of an odd integer (π/2, 3π/2, 5π/2, . . . ).
Further, according to the conventional DFB semiconductor laser shown in FIG. 22, the difference (n0−n1) has a constant value due to a fixed difference in cross-sectional shape between the connecting waveguide 7 and the diffraction grating waveguides 6a, 6b. 
As a result, with the beam wavelength λ0 being 2π, the length L of the connecting waveguide must be set so as to be closest to one of the values given by π multiplied by a half value of an odd integer (π/2, 3π/2, 5π/2, . . . ), which means that the value of length L of the connecting waveguide cannot be selected optionally.
Now, if the length L of the connecting waveguide cannot be selected optionally in the DFB semiconductor laser which has the diffraction grating waveguides 6a, 6b separated from each other in the beam outputting direction, the following problems occur.
First, if the length L of the connecting waveguide cannot be selected freely, this restriction affects the overall size and shape of the DFB semiconductor laser and accordingly, the freedom of the device design is limited.
Second, if the length L of the connecting waveguide is short, intensity of the beam outputted by the DFB semiconductor laser is low.
Specifically, as shown in FIGS. 24A and 24B, if the length L of the connecting waveguide is short, the beam generated in this DFB semiconductor laser has an intensity distribution characteristic represented by a curve which looks somewhat like a mountain or a wave.
This means that beam output intensity P0 of the beam 4 outputted from the two ends of the DFB semiconductor laser is decreased.
Furthermore, since spatial hole burning is induced, degradation of intensity of the outputted beam or beam spectrum width can often be induced.
To the contrary, if the length L of the connecting waveguide is long, the overall size of the DFB semiconductor laser is increased, and the number of devices manufactured per wafer is decreased, so that the manufacturing cost of the device is increased.
Further, if observed in detail, in the DFB semiconductor laser, discontinuity occurs in the effective refractive index n on the boundary surface between the diffraction grating waveguides 6a, 6b and the connecting waveguide 7.
Therefore, part of the beam 4 generated in the active layer 2 and propagated on the diffraction grating waveguides 6a, 6b and the connecting waveguide 7 in the direction of cavity is reflected by this boundary surface.
As a result there is a problem that it is difficult to achieve “single-mode oscillation” described above.